Lipid droplets (LDs) are ubiquitous organelles in eukaryotic cells that organize and store cellular lipids, including triacylglycerols (TGs), which provide stores of energy and sources of membrane lipids. Despite recent advances in understanding LD biology in cellular physiology and disease, most fundamental key questions about LDs remain unanswered. In this proposal, we focus on the most basic question of determining the mechanisms underlying LD formation. To do so, Aims 1 and 2 will employ a novel system we developed, in which LD formation is induced on a null background in mammalian cells. We accomplish this by inducing acyl CoA: diacylglycerol acyltransferase (DGAT) 1 or DGAT2, which catalyze TG synthesis and lie just upstream to the process of LD formation. By inducing LD formation in cels, we will address key questions: How is TG synthesis coordinated with newly forming LDs? Where in the cell do LDs form? How do monolayer-bound LDs form from bilayer membranes? Specifically, Aim 1 focuses on determining the localization and activity of DGAT enzymes before, during, and at later stages of LD formation. Aim 2 focuses on determining where LD formation occurs in the cell, how LD formation relates to membrane topology, tubular (or curved) ER, and how TGs in the ER are organized during LD formation. The final aim focuses on our findings that specific proteins, such as the hepatitis C Core protein, are apparently targeted to newly forming LDs. Aim 3 therefore will elucidate the mechanism that underlies the targeting of Core to LDs and will identify other endogenous cellular proteins that gain access to LDs through this mechanism. Our studies will provide fundamental knowledge about LDs and cellular lipid metabolism. Additionally, a better understanding of LD formation may have therapeutic implications for diseases of LD excess, such as obesity, diabetes, and atherosclerosis, and may have practical applications in engineering plants and microorganisms for increased oil production.